The emission of light at longer wavelengths after absorption of incident photons by chromophores is termed fluorescence. Under certain conditions, when human skin is illuminated with ultraviolet or visible light, cutaneous fluorescence can be detected. This phenomenon may be termed "autofluorescence" when it occurs in the absence of exogenously administered fluorescent compounds (Anderson, 1982; Zeng, 1993a).
Autofluorescence is believed to originate from various endogenous fluorophores, including nicotinamide adenine dinucleotide, elastin, collagen, flavins, amino acids and porphyrins. Porphyrins are naturally produced as intermediates in the biosynthetic pathway of heme. Protoporphyrin IX is the immediate precursor of heme. Spectrophotometry may be useful for distinguishing between porphyrins and other endogenous fluorophores based on the emission spectra of the fluorescent light.
Autofluorescence emission spectra and fluorescence images can be generated and recorded when incident light is shone on skin. An ultraviolet A emitting Wood's lamp may be used to assess cutaneous fluorescence for dermatologic diagnosis (Kochevar et al 1993). Macrospectrophotometry is a simple, flexible and efficient method of detecting cutaneous porphyrins illuminated with appropriate wavelengths of light; the technique is painless, takes only a few seconds and does not require a skin biopsy (Zeng et al., 1993).
Autofluorescence photographic images have been used to evaluate treatment responses in acne (Lucchina et al 1996, Martin R. J. et al 1973). Analysis and comparison of emission spectra has also been studied as a noninvasive diagnostic tool for skin diseases (Zeng et al 1995; Lohman 1988, Steremborg et al 1995).
Punctate red fluorescence on the nose and forehead under Wood's lamp illumination was reported as early as 1927 (Bommer, 1927), and has been linked to the presence in acne of porphyrins generated by Propionibacterium acnes (Cornelius, 1967; McGinley, 1980; Lee et al 1978; Konig et al, 1992; Johnson, 1987; Lucchina, 1996). The presence of red skin autofluorescence at the centre of experimentally produced or grafted tumours has been reported for rats (Policard, 1924; Gougerot, 1939; Rochese, 1954), mice (Konig, 1989), rabbits (Ghadially, 1960) and for chemically-induced squamous cell carcinoma in the cheek pouch of the hamster (Harris, 1987). In the latter case, the tumours were examined microscopically and the red fluorescence was shown to be restricted to the surface keratin layer (Harris, 1987). Similar red autofluorescence has also been reported for human oral and oropharyngeal squamous cell carcinoma (Harris, 1987; Dhingra, 1996; Konig, 1994), dysplastic areas of the oral mucosa (Ingrams et al., 1997) as well as normal human tongue (Harris et al, 1987).
In studies where spectroscopic analysis was performed, the fluorescence emission peak was centred around 636-640 nm (Konig et al., 1994; Dhingra, 1996). Konig et al. (1994) believed it was related to bacterial synthesis of porphyrins whereas Dhingra et al (Dhingra, 1996) hypothesized that the red autofluorescence could be caused by a build-up of endogenous porphyrins by tumor cells.
Macrospectrophotometry may be used to detect skin porphyrin in patients receiving exogenous porphyrins, or porphyrin precursors, for photodynamic therapy, and to follow the time course accumulation of porphyrins in photodynamic therapy (Lui, 1996; Rhodes, 1997; Stringer, 1996). The intensity of the fluorescence emission peaks has been shown to correlate with the amount of exogenous porphyrin precursor applied on the skin (Rhodes, 1997).
Porphyrins that absorb light may induce photochemical reactions that can be toxic to living cells. Such toxicity may be due to the local generation of reactive oxygen species (Arakane et al., 1996). The toxicity generated by light activation of pharmacologically elevated levels of porphyrins is the basis for photodynamic therapy which may be used to treat a variety of conditions, including cancer (see U.S. Pat. Nos. 5,211,938; 5,234,940; 5,079,262; all to Kennedy et al.) The low levels of natural porphyrins present in most tissues are not known to cause deleterious photochemical effects (Goerz et al., 1995).
Very low levels of porphyrins have been shown to be present in biopsies of human skin containing the epidermis and dermis (Goerz et al., 1995; Pathak 1963), as well as whole epidermis isolated by suction blister (Gog, 1973). Protoporphyrin IX was the predominant porphyrin type (Goerz et al., 1995; Gog, 1973) except in porphyria cutanea tarda patients, where uroporphyrin was predominant (Malina, 1978). Goerz et al., 1995, report that skin does not normally contain sufficient levels of porphyrins to allow one to perform photodynamic therapy, and consequently photodynamic therapy requires exogenous addition of photosensitizer.
Psoriasis is a hyperproliferative and inflammatory disease characterized by red scaly plaques on the skin. Current accepted or experimental methods of using light to treat psoriasis involve either the use of potentially carcinogenic ultraviolet light alone or the administration of exogenous photosensitizer, or a precursor of a photosensitizer (such as aminolevulinic acid), followed by light exposure. Blue and red light can activate protoporphyrin IX and this has been used to improve psoriasis when exogenous aminolevulinic acid (ALA) is applied on the skin to induce protoporphyrin IX synthesis (Boehncke, 1994; Nelson, 1995). However, photosensitizers or photosensitizer precursors such as ALA may have adverse side effects when administered topically or systemically. For example, they may induce photosensitivity on clinically normal skin.